Crystallizer temperature control via solid additive control

ABSTRACT

A method of crystallizing polymeric pellets includes a step in which a plurality of polymeric pellets are introduced into a crystallizer. While within the crystallizer, the plurality of pellets is contacted with a plurality of solid additives for the purpose of adjusting the average pellet temperature. The plurality of solid additives adjusts the temperature of the pellets by having a temperature sufficient to allow at least partial crystallization of the plurality of polymeric pellets while maintaining the average pellet temperature below the melting temperature. Finally, the plurality of polymer pellets is removed from the outlet of the crystallizer.

FIELD OF THE INVENTION

The present invention relates generally to methods and systems ofcrystallizing polymer pellets and more specifically to methods andsystems of crystallizing polyester pellets.

BACKGROUND OF THE INVENTION

Thermoplastic resins are used in a multitude of commercial applications.Polyesters such as polyethylene terphthalate (PET),polyethylenenaphthalate (PEN), and similar polymers and copolymers, inparticular, have become staple commodities whose manufacture is wellknown and mature. Applications of polyesters includes food, beverage,and other liquid containers as well as synthetic fibers. Severalpolyesters such as PET may exist both in amorphous and semi-crystallineforms. Amorphous PET is transparent while crystalline PET is opaque.

In the conventional PET process, PET is formed by esterification ofterephthalic acid and ethylene glycol in a reaction vessel to form apre-polymeric mixture. The esterification need not be catalyzed. Thepre-polymeric paste is subsequently heated to promote polymerization.The resulting mixture is then subjected to polycondensation in a melt atelevated temperatures, for example, 285° C., in the presence of asuitable catalyst. Compounds of Sn, Sb, Ge, Ti, or others have been usedas polycondensation catalysts. The polymer is extruded directly from thepolycondensation reactor into strands. The hot, extruded strands arecontacted with cool water prior to chopping into pellets, dried, andstored into silos prior to crystallizing.

Pelletizing processes wherein strands are stretched prior to pelletizingare disclosed in U.S. Pat. No. 5,310,515. Conventional wisdom dictatesthat at least the surface of the pellets must be cooled to 20° C. to 30°C. to avoid sintering during storage. During storage, heat from thehotter interior of the pellets is distributed throughout the pellets.Thus, warm pellets, i.e., pellets whose exterior is significantly higherthan 20° C. to 30° C. might agglomerate during storage followingtemperature equilibration. In addition to the decrease in temperaturebrought about by contact with water, the pellets can be further cooledto the desired temperature with cool air, nitrogen, or inert gas. Thepellets are stored, and then subsequently reheated to the desiredcrystallization temperature. These steps of heating, cooling, andreheating result in a significant energy penalty in an already energyintensive process. The crystallization of the hot pellets can beaccomplished in a crystallizing shaker and fluid bed. Solid stating isused to both raise inherent viscosity and remove acetaldehyde.

With reference to FIGS. 1A, 1B, and 1C, diagrams of PET manufacturingfacilities are provided. PET processing facility 10 includes mixing tank12 in which terephthalic acid (“TPA”) and ethylene glycol (“EG”) aremixed to form a pre-polymeric paste. This pre-polymeric paste istransferred and heated in esterification reactor 14 to form anesterified monomer. The pressure within esterification reactor 14 isadjusted to control the boiling point of the ethylene glycol and helpmove the products to esterification reactor 16. The monomer fromesterification reactor 14 is subjected to additional heating inesterification reactor 16 but this time under less pressure than inesterification reactor 14. Next, the monomers from esterificationreactor 16 are introduced into pre-polymer reactor 18. The monomers areheated while within pre-polymer reactor 18 under a vacuum to form apre-polymer. The inherent viscosity of the pre-polymer begins toincrease within pre-polymer reactor 18. The pre-polymer formed inpre-polymer reactor 18 is sequentially introduced into polycondensationreactor 20 and then polycondensation reactor 22. The pre-polymer isheated in each of polycondensation reactors 20, 22 under a larger vacuumthan in pre-polymer reactor 18 so that the polymer chain length and theinherent viscosity are increased. After the final polycondensationreactor, the PET polymer is moved under pressure by pump 24 throughfilters 26, 28 and through dies 30, 32, 34, forming PET strands 36, 38,40 (see FIG. 1B).

With reference to FIG. 1B, a method for forming polyester pellets isillustrated. Extruded polymer strands 36, 38, 40 are cooled by waterspray streams 42, 44, 46 onto the strands as the strands emerge fromdies 30, 32, 34. After emerging from dies 30, 32, 34, strands 36, 38, 40are cut by cutters 54, 56, 58 into pellets 48, 50, 52 while the strandsare still hot. Polyester pellets formed in this manner tend to have acylindrical shape, but can be modified to cubic, dog bone, or othershapes. At this point in the process, polyester pellets are usuallyamorphous. The polyester pellets are typically crystallized before beingshipped to a customer. Such crystallization allows subsequent drying athigher temperatures so that the polyester may be extruded as desired.Crystallization of the polyester pellets is typically achieved byreheating the pellets to a temperature above the crystallizationtemperature. As the pellets crystallize, additional heat is derived dueto the generated heat of crystallization. This additional heat tends tomake the pellets soft and adherent to each other. Therefore, the pelletsare agitated to avoid them sticking together due to softening. Aftercrystallization, the pellets are generally solid stated to raiseinherent viscosity with inert gas passing around the hot pellets.

With reference to FIG. 1C, a schematic of an alternative pellet formingprocess is provided. In this variation, strands 60, 62, 64 emerging fromdie dies 66, 68, 70 are cut into pellets 72, 74, 76 under water by dieface cutters 80, 82, 84. In this variation, the extruded polyesterstrands are completely immersed and cut underwater upon exiting dies 66,68, 70. Pellets 72, 74, 76 formed in this manner tend to have aspherical shape because of the surface tension of the molten polyesterwhen emerged in water. Initially, after cutting, pellets 72, 74, 76still retain a substantial amount of heat in the interior. Subsequently,the pellet/water combination is sent to dryer 90 via conveying system92. Examples of useful dryers include centripetal dryers that removepellets 72, 74, 76 from the water. Upon exiting dryer 90, additionalwater is boiled off due to the heat content of pellets 72, 74, 76, whichis still high upon emerging from dryer 90. If the pellet/watercombination is transported to the dryer sufficiently fast the polyesterpellets may retain sufficient heat for crystallization to occur. Pellets72, 74, 76 are then transferred to crystallizer 94 where they reside fora residence time (about 2 to 20 minutes) for crystallization to occur.Crystallizer 94 also provides sufficient agitation to inhibit thepolyester pellets from sticking together.

International Patent Appl. No. WO2004/033174 and U.S. Pat. Appl. Nos.20050110182 and 20050110184 disclose methods for crystallizing polymericpellets. International Patent Appl. Nos. WO2004/033174 discloses amethod in which polymeric pellets are treated in a liquid bath (e.g.,water bath) at an elevated temperature to induce crystallization. U.S.Pat. Appl. Nos. 20050110182 and 20050110184 disclose method in which airis injected into the pellet/water slurry of FIG. 1C in order totransport the pellets quickly to and through dryer 90.

After crystallization, pellets 72, 74, 76 are transported by dense phaseconvey system 96 to one or more pellet processing stations. Such densephase convey systems utilize air to move the pellets from one locationto another. For example, the pellets are transported to a blending siloin which the average properties of the pellets might be adjusted. Insuch blending silos, polyester pellets are mixed together to achieve atarget specification. Such specification may be with respect to color,molecular weight, catalyst concentration, additive concentration,density, and the like. In still another example, the pellets areconveyed to a solid stating process reactor. It should be noted, thatdense phase convey systems tend to be more useful than dilute phaseconvey systems in this application since dilute phase convey systems canresult in the surface of the pellets being melted or have high impactvelocities thereby forming undesirable streamers and fines.

Although these methods and systems for making polymeric pellets and, inparticular, polyester pellets work well, the equipment tends to beexpensive to fabricate and to maintain. A typical PET manufacturing linemay include several crystallizers each of which utilizes a rather largemotor and occupies a larger footprint in the manufacturing plant. Theinitial capital investment of such crystallizer may easily exceed amillion dollars.

Accordingly, there exists a need for polymer processing equipment andmethodology that is less expensive to install, operate, and maintain.

SUMMARY OF THE INVENTION

The present invention overcomes one or more problems by providing in atleast one embodiment a method of crystallizing polymeric pellets. Themethod of the present embodiment includes a step in which a plurality ofpolymeric pellets are introduced into a crystallizer. Forcrystallization to be possible, the polymeric pellets must be formedfrom one or more polymers that are crystallizable. Such crystallizablepolymers are characterized by a crystallization temperature and amelting temperature. Moreover, the plurality of polymeric pellets ischaracterized with an average pellet temperature. The plurality ofpolymeric pellets is introduced into the crystallizer with an initialaverage temperature. While within the crystallizer, the plurality ofpellets is contacted with a plurality of solid additives for the purposeof adjusting the average pellet temperature. The plurality of solidadditives is introduced into a contact region within the crystallizer.The plurality of solid additives adjusts the temperature of the pelletsby having a temperature sufficient to allow at least partialcrystallization of the plurality of polymeric pellets while maintainingthe average pellet temperature below the melting temperature. Finally,the plurality of polymer pellets is removed from the outlet of thecrystallizer.

In another embodiment of the present invention, a crystallizer forcrystallizing polymeric pellets is provided. The crystallizer of thisembodiment includes an inlet for receiving a plurality of polymericpellets and an outlet for removing the pellets. The crystallizer furtherincludes a solid additive applicator to contact the plurality ofpolymeric pellets with a plurality of solid additives. The crystallizeralso includes a conveyor for transporting the plurality of pellets froma first location to a second location. In a variation of thisembodiment, the conveyor vibrates the pellets in such a manner such thatthe pellets move towards the outlet. Advantageously, the conveyor alsoagitates the pellets during conveying so that sticking or clumpingtogether of the pellets is minimized.

Additional advantages and embodiments of the invention will be obviousfrom the description, or may be learned by practice of the invention.Further advantages of the invention will also be realized and attainedby means of the elements and combinations particularly pointed out inthe appended claims. Thus, it is to be understood that both theforegoing general description and the following detailed description areexemplary and explanatory of certain embodiments of the invention andare not restrictive of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of a polyester manufacturing linethrough the polycondensation reactors;

FIG. 1B is a schematic illustration of a polyester manufacturing lineshowing processing after polycondensation reactors using strand cuttersto form the polyester pellets;

FIG. 1C is a schematic illustration of a polyester manufacturing lineshowing processing after polycondensation reactors using die facecutters to form the polyester pellets;

FIG. 2 is a schematic illustration of an embodiment of a crystallizerusable in the methods of the invention;

FIG. 3 is a schematic illustration of an embodiment of a crystallizerwith an open top;

FIG. 4 is a schematic illustration of an embodiment of a crystallizer inwhich the solid additives are recycled;

FIG. 5A is a schematic illustration of an embodiment using a magneticfield to separate the solid additives from the pellets;

FIG. 5B is a schematic illustration of an embodiment using densitydifferences between the pellets and the solid additives to separate thesolid additives from the pellets;

FIG. 5C is a schematic illustration of another embodiment using densitydifferences between the pellets and the solid additives to separate thesolid additives from the pellets;

FIG. 5D is a schematic illustration of an embodiment using dragdifferences between the pellets and the solid additives to separate thesolid additives from the pellets;

FIG. 5E is a schematic illustration of another embodiment using dragdifferences between the pellets and the solid additives to separate thesolid additives from the pellets;

FIG. 5F is a schematic illustration of an embodiment using sizedifferences between the pellets and the solid additives to separate thesolid additives from the pellets;

FIG. 5G is a schematic illustration of another embodiment using sizedifferences between the pellets and solid additives to separate thesolid additives from the pellets;

FIG. 5H is a schematic illustration of an embodiment using densitydifferences between the pellets and the solid additives to separate thesolid additives from the pellets with a cyclone;

FIG. 5I is a schematic illustration of another embodiment using densitydifferences between the pellets and the solid additives to separate thesolid additives from the pellets with a cyclone;

FIG. 6A is a schematic illustration of an embodiment of a crystallizerusing partitions to assist in conveying the polymeric pellets;

FIG. 6B is a schematic illustration of an embodiment of a crystallizerusing a spiraling motion to convey the polymeric pellets;

FIG. 7 is a side view of a crystallizer system using the crystallizer ofFIG. 6A to convey the polymeric pellets;

FIG. 8A is a side view of a crystallizer system using the crystallizerof FIG. 6B to convey the polymeric pellets; and

FIG. 8B is a front view of a crystallizer system using the crystallizerof FIG. 6A to convey the polymeric pellets.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to presently preferredcompositions, embodiments and methods of the present invention, whichconstitute the best modes of practicing the invention presently known tothe inventors. The Figures are not necessarily to scale. However, it isto be understood that the disclosed embodiments are merely exemplary ofthe invention that may be embodied in various and alternative forms.Therefore, specific details disclosed herein are not to be interpretedas limiting, but merely as a representative basis for any aspect of theinvention and/or as a representative basis for teaching one skilled inthe art to variously employ the present invention.

Except in the examples, or where otherwise expressly indicated, allnumerical quantities in this description indicating amounts of materialor conditions of reaction and/or use are to be understood as modified bythe word “about” in describing the broadest scope of the invention.Practice within the numerical limits stated is generally preferred.Also, unless expressly stated to the contrary: percent, “parts of,” andratio values are by weight; the term “polymer” includes “oligomer,”“copolymer,” “terpolymer,” and the like; the description of a group orclass of materials as suitable or preferred for a given purpose inconnection with the invention implies that mixtures of any two or moreof the members of the group or class are equally suitable or preferred;description of constituents in chemical terms refers to the constituentsat the time of addition to any combination specified in the description,and does not necessarily preclude chemical interactions among theconstituents of a mixture once mixed; the first definition of an acronymor other abbreviation applies to all subsequent uses herein of the sameabbreviation and applies mutatis mutandis to normal grammaticalvariations of the initially defined abbreviation; and, unless expresslystated to the contrary, measurement of a property is determined by thesame technique as previously or later referenced for the same property.

It is also to be understood that this invention is not limited to thespecific embodiments and methods described below, as specific componentsand/or conditions may, of course, vary. Furthermore, the terminologyused herein is used only for the purpose of describing particularembodiments of the present invention and is not intended to be limitingin any way.

It must also be noted that, as used in the specification and theappended claims, the singular form “a”, “an”, and “the” comprise pluralreferents unless the context clearly indicates otherwise. For example,reference to a component in the singular is intended to comprise aplurality of components.

Throughout this application, where publications are referenced, thedisclosures of these publications in their entireties are herebyincorporated by reference into this application to more fully describethe state of the art to which this invention pertains.

The term “polymeric pellet” as used herein means a three-dimensionalobject formed from a polymer. Such polymeric objects include a largestdimension that is greater than or equal to the extent of the polymericobject in any direction. Polymeric pellets occur in a number of shapessuch as spherical, cylindrical, and the like. The largest dimension of asphere is the diameter.

The term “heat of crystallization” as used herein means the quantity ofheat released as a unit of mass of a substance crystallizes.

The term “crystallization temperature” as used herein means thetemperature at which at least part of a material begins to crystallize.

The term “melting temperature” as used herein means the temperature atwhich at least part of a material is transformed from a crystallinestate to a liquid. When a material undergoes such a transformation overa temperature range, for purposes of the present invention, the meltingtemperature is the median temperature of such a range. Typically,amorphous pellets melt at a lower temperature than crystalline pellets.

The term “degree of crystallinity” as used herein means the fractionalamount of crystallinity in a polymeric sample. In the present invention,the degree of crystallinity is the average fractional amount ofcrystallinity in the polymeric pellets. Degree of crystallinity can beexpressed as either a weight percent or a volume percent. As usedherein, the degree of crystallinity is expressed as a weight percentunless expressly stated to be otherwise. The degree of crystallinity canbe determined by differential scanning calorimetry (“DSC”).

The term “cyclone” as used herein means a vortex separator that usesrotational effects and gravity to separate mixtures of solids and/orfluids.

In an embodiment of the present invention, a method of crystallizing aplurality of polymeric pellets is provided. The plurality of polymericpellets in accordance with the present invention is introduced into acrystallizer. In general, the plurality of pellets to be crystallized inaccordance with the present invention are amorphous pellets or pelletswith less than a desired degree of crystallinity. In a variation of thepresent embodiment, the degree of crystallinity of the pellets prior tocrystallization is less than 30 wt. %. In another variation of thepresent embodiment, the degree of crystallinity of the pellets prior tocrystallization is less than 20 wt. %. In still another variation of thepresent embodiment, the degree of crystallinity of the pellets prior tocrystallization is less than 10 wt. %.

After crystallization, the degree of crystallinity is typically greaterthan 30 wt. %. In other variations, after crystallization, the degree ofcrystallinity is greater than 40 wt. %. For most applications, thedegree of crystallinity after crystallization is less than 70 wt. %. Inother variations, the degree of crystallinity after crystallization isless than 60 wt. %. In still other variation, the degree ofcrystallinity after crystallization is less than 50 wt. %.

Advantageously, the polymeric pellets that are to be crystallizedinclude any crystallizable polymer. The crystallizable polymers arecharacterized by a crystallization temperature and a meltingtemperature. Examples of such polymers include, but are not limited to,polyesters, polyolefins, polystyrenes, nylons, and polyketones. Polymersthat have a relatively high heat of crystallization are most useful. Ina variation, the heat of crystallization of such useful polymers is suchthat the heat of crystallization divided by the heat capacity of thepolymer is at least 5° C. A crystallization temperature and a meltingtemperature additionally characterize the crystallizable polymers. Thepresent embodiment is particularly useful for crystallizing polyalkyleneterephthalate polymers, especially polyethylene terephthalate polymers.

The polyalkylene terephthalate polymers exiting the melt phasepolymerization process, or as introduced into the crystallizer orcrystallization zone, have an It.V. of at least 0.50 dL/g, or at least0.55 dL/g, or at least 0.6 dL/g, and especially at least 0.70 dL/g, orat least 0.72 dL/g, or at least 0.74 dL/g, or at least 0.76 dL/g, or atleast 0.78 dL/g, and up to about 1.2 dL/g, or 1.1 dL/g, or 0.9 dL/g. Thepartially crystallized polyester polymers are also advantageously notsolid state polymerized. Thus, there is also provided an embodimentencompassing a shipping container containing a plurality of partiallycrystallized pellets with a degree of crystallinity of at least 20% andan It.V. of at least 0.70 dL/g which have not been solid statepolymerized. Suitable shipping containers are those suitable forshipping in commerce, having a volume of at least 1 cubic meter or more,or 2 cubic meters or more, or 3 cubic meters or more, or 8 cubic metersor more, or 20 cubic meters or more, and include Gaylord boxes, railtank cars, trailers for tractor trailers, and ship hulls. The It.V. ofthe pellets may any value identified above higher than 0.70 dL/g, andthe degree of crystallinity may be any value higher than 20% asidentified above.

The pellets utilized in the methods of the invention are formed from avariety of methods know to those skilled in the art. Examples of suchpellet forming processes include, but are not limited to, the processesdepicted in FIGS. 1A, 1B, and 1C and described above. It should berecognized that the present invention provides, in at least oneembodiment, an improvement over the crystallizer described in connectionwith the description of FIG. 1C. In particular, the present inventionallows for a reduction in length of such crystallizers (i.e., along thedirection that the pellets are conveyed) along with a concurrentsignificant decrease in equipment cost. The methods of the presentembodiment are used to crystallize pellets of virtually any shape orsize. Typically, at least a portion of the plurality of polymericpellets includes three-dimensional objects characterized by a largestsize dimension, which is less than 0.25 inches. Examples of shapedpellets that are usable in the practice of the present inventioninclude, but are not limited to, spherically shaped pellets,cylindrically shaped pellets, and pellets with a rectangular crosssection.

With reference to FIG. 2, a schematic illustration of an embodiment ofthe present invention is provided. The method of this embodimentcomprises introducing a plurality of polymeric pellets 100 intocrystallizer 102 via pellet inlet 104. In a variation, plurality ofpellets 100 is introduced into the crystallizer in an amount of 5,000lb/hr to 200,000 lb/hr. In a variation, recycled pellets may beintroducing into crystallizer 102 along with polymeric pellets 100 viainlet 104. In this variation, the temperature of pellets 100 may beadjusted by varying the temperature of the recycled pellets.

The plurality of polymeric pellets 100 has an initial average pellettemperature when introduced into crystallizer 102. In some variations ofthe present embodiment, pellets 100 are at an elevated temperature thatis useful for crystallization to occur while pellets 100 are resident incrystallizer 102. In at least some embodiments, such as when the pellets100 are PET, the elevated temperature is from 135° C. to 205° C., and inother embodiments from 150° C. to 200° C. The polymeric pellets may beprovided in any manner including processes in which the polymericpellets are reheated after cooling. An example of such a processincludes PET strands cut by a strand cutter as set forth above inconnection with the description of FIG. 1B.

In a particularly useful variation of the present embodiment, polymericpellets 100 are cut by die face pellet cutters as set forth above inconnection with the description of FIG. 1C. In this variation, pellets100 are transferred from the die face pellet cutters to dryer 90 viapellet conveying system 92. Examples of useful dryers includecentripetal dryers that remove pellets 100 from the water. It should beappreciated that in this context dryer 90 is any device that may be usedto separate the pellets from the water. Upon exiting dryer 90,additional water may be boiled off due to the heat content of pellets100, which is still relatively high upon emerging from dryer 90. In thisvariation using die face pellet cutters, pellets 100 are transferredsufficiently quickly from the cutters to the dryer so that pellets 100retain a substantially amount of heat. Typically, the polymeric pelletsexiting such a dryer have temperatures exceeding 135° C.

It should be appreciated that each of pellets 100 typically has arelatively non-uniform temperature distribution with the interior ofpellets 100 being hotter than the exterior of the pellets. This is dueto the cooling effect of the water used in the die face cutters andpellet conveying system 92 and the low thermal conductivity of thepolymer. Moreover, each pellet is likely to have a slightly varyingtemperature profile. Therefore, it is appropriate to describe theplurality of pellets as having an average pellet temperature.

It should also be appreciated that the water used to transport pellets100 from the die face cutters to dryer 90 may be substituted by otherconveying fluids with superior or more desirable heat transferproperties. The average temperature of pellets 100 may also becontrolled by the temperature of the water (or other conveying fluid)used to convey pellets 100 from the die face cutters to dryer 90. Forexample, the conveying fluid may be heated to allow for higher initialaverage pellets temperatures (introduced to crystallizer 102) or cooledto allow for lower initial average pellet temperatures. In a typicalpolyester forming process, the transit time from the die face cutters todryer 90 is on the order of a few seconds with a pellet containingslurry traveling at a velocity from 10 to 30 feet/s while in pelletconveying system 92.

Crystallizer 102 conveys the plurality of pellets 100 along longitudinaldirection d₁ from inlet 104 to pellet outlet 106 along conveyor 108.While being conveyed by the crystallizer 102, pellets 100 can beagitated to help prevent clumping or sticking together of pellets 100 asthe average pellet temperature increases during crystallization becauseof liberation of the heat of crystallization. In at least oneembodiment, motor 110 in contact with conveyor 108 by shaft 112 canprovide such agitation. Such agitation may cause shaking or vibration ofpellets 100. In general, conveyor 108 includes bottom wall 114, endwalls 115, 116 and opposed side walls (not shown). Crystallizer 102 mayalso include optional top 118, which is positioned atop conveyor 108 toform cavity 119. In a further refinement of the invention, suchagitation can also assist in conveying pellets 100 along direction d₁.

Pellets 100 are removed from the crystallizer 102 via pellet outlet 106and transferred to the next processing or storage apparatus. Theresidence time of the pellets 100 within crystallizer 102 can vary independence upon many factors, such as, the type of polymer beingcrystallized, the initial average pellet temperature, the throughput ofpellets being processed, and the like. Typically, residence times arefrom 1 second to 1 hour. In other variation, the residence time is from1 minute to 10 minutes.

Still referring to FIG. 2, polymeric pellets 100 are contacted withsolid additives 120 to adjust the average temperature of pellets 100.Solid additives 120 are introduced into contact region 122 ofcrystallizer 102 via solid additive applicator 124. Suitable examples ofsolid additive applicators include, but are not limited to, loss inweight feeders, rotary air locks, vibrating distributors, and the like.

Pellets 100 are contacted with the solid additives 120 thereby havingheat either transferred to or removed from the pellets. Solid additives120 have a temperature sufficient to allow (by adjusting the pellettemperature) at least partial crystallization of the plurality ofpolymeric pellets 100 while maintaining the average temperature of theplurality of pellets below the melting temperature while the pluralityof pellets is within the crystallizer 102. The temperature of theplurality of pellets 100 is advantageously adjusted in order to controlthe rate of crystallization. The higher the average temperature ofpellets 100 the higher the rate of crystallization. If pellets 100 aretoo cold (i.e., below 135° C.), it can be relatively difficult to supplysufficient additional heat to achieve crystallization. If pellets 100are too hot, the pellets may start to melt due to the heat ofcrystallization liberated as the pellets crystallize. The presentembodiment of the invention advantageously allows the average pellettemperature to be optimized so as to minimize the length of thecrystallizer 102 because a higher initial average pellet temperature maybe used with cooling as provided in the present invention inhibitingheating caused by the liberated heat of crystallization. Minimizing thelength of crystallizer 102 reduces expenses associated with the purchaseand maintenance of such crystallizers, which tend to be expensive.

Solid additives 120 used in the practice the invention comprisethree-dimensional objects characterized by a largest size dimension. Inat least one embodiment, the largest size dimension of these objects isless than 0.25 inches. Moreover, in certain embodiments, it may beadvantageous for the objects to have a size that is compatible withefficient heat transfer from or to the plurality of polymeric pellets100. Therefore, in a variation of the present embodiment, thethree-dimensional objects have a surface to volume ratio from 20:1 to500:1. In another variation of the present embodiment, thethree-dimensional objects have a surface to volume ratio from 50:1 to400:1. In still another variation of the present embodiment, thethree-dimensional objects have a surface to volume ratio from 100:1 to500:1.

Suitable, examples of such objects include, but are not limited to,metallic objects, ceramic objects, glass objects, polymeric objects, andcombinations thereof. In a refinement, the objects are metallic objectsthat are magnetic. In a particularly useful embodiment, thethree-dimensional objects are recycled polymeric objects and inparticular, objects made from recycled polyester. Also, any shapedobject is useable in the present embodiment. Many objects useable as thesolid additive are spherical or cylindrical. In another variation, thesolid object is a solid that sublimes. Examples of such solid objectsinclude, but are not limited to, iodine, carbon dioxide, and the like.When sublimeable solids are used, crystallizer 102 optionally includes avent for removing vapors generated during sublimation.

In one variation of the present embodiment, polymer pellets 100 entercrystallizer 102 with an average temperature that is above an optimaltemperature for crystallization. In this variation, the polymer pellets100 are cooled by the solid additives, which in this variation will havea temperature lower than that of the average temperature of the pellets100. The method of the present variation is particularly useful for thecrystallization of polyethylene terephthalate pellets, which usuallystart to crystallize at a temperature of 135° C. and melt at atemperature of 200° C. For every 10° C. increase in the averagetemperature of the polyethylene terephthalate pellets that entercrystallizer 102, the length l₁ of crystallizer 102 can be optimallyreduced by a factor of two if sufficiently cooling in accordance withthe present invention is utilized. When pellets 100 have regions withsufficient heat content for crystallization to occur, the average pellettemperature increases as pellets 100 are conveyed along direction d₁.This temperature increase is the result of the liberation of the heat ofcrystallization from pellets 100 as they crystallize.

In one refinement of the present variation, the difference between theinitial average pellet temperature (as introduced into crystallizer 102)and the crystallization temperature is less than the temperature riseinduced by the crystallization of pellets 100 in the absence of externalcooling. Therefore, in this refinement, cooling is applied to thepellets 100 via comingling of the solid additives 120 with the pelletsat the point before an average temperature conducive to melting orsticking of pellets 100 occurs but after crystallization commences.

In a further refinement of the present variation, one or more sides ofcrystallizer 102 are partially or completely insulated by insulation asschematically illustrated at 130. If more cooling is needed, a lesseramount or no insulation can be provided. Additional cooling ofcrystallizer 102 could also be realized by removing the top 118 of thecrystallizer as illustrated in FIG. 3. In this variation, crystallizer102 is of a construction without a top section. Even more cooling may beprovided by directing air over the pellets with a fan. Such acrystallizer is usable where the pellets are to be used in applicationsallowing exposure of the pellets to ambient conditions.

In another variation of the present invention, the pellets will containsufficient internal heat for crystallization to occur. In othervariations of the present invention, the pellets do not containsufficient heat for crystallization. In this latter variation, theaverage pellet temperature is adjusted by contact with solid additivesat elevated temperature and optionally introduced closer to inlet 104.

In yet another variation of the present embodiment, polymer pellets 100enter crystallizer 102 with an average temperature too low forcrystallization to proceed to a desired degree. In this situation, thetemperature of the solid additives is such that polymeric pellets areheated by contacting with the solid additives (i.e., the temperature ofthe solid additives is higher than the average temperature of pellets100).

In still another refinement of the present variation, polymeric pellets100 are introduced to crystallizer 102 with a sufficient amount of heatso that there are regions in pellets 100 having a temperature greaterthan or equal to the crystallization temperature of the polymer fromwhich pellets 100 are formed. Such a temperature results in at leastpartial crystallization while pellets 100 are within crystallizer 102.

In still another variation of the present embodiment, polymer pellets100 enter crystallizer 102 with an average temperature too low forcrystallization to proceed to a desired degree. In this situation, thetemperature of the solid additive 120 is such that polymeric pellets 100are heated by contact with the solid additive (i.e., the temperature ofthe solid additive is higher than the average temperature of pellets100).

With reference to FIG. 4, a schematic illustration of a variation inwhich the solid additives 120 are recycled is provided. In thisvariation, crystallizer system 140 includes crystallizer 102. Bothpellets 100 and solid additives 120 are removed from the crystallizervia pellet outlet 106 and directed into separator 142, which separatespellets 100 from solid additives 120. Solid additives 120 are thendirected to temperature adjusting device 144, which either heats orcools solid additives 120 as desired. Next, solid additives 120 arere-introduced into crystallizer 102 via solid additive applicator 124.This process is advantageously successively repeated as desired.

In at least certain embodiments, the method of the present embodimentfurther includes the step of separating solid additives 120 from thepolymeric pellets 100. FIGS. 5A-5I provide examples of methods forseparating solid additives 120 from plurality of pellets 100. FIG. 5Aprovides an illustration of the separation of solid additives 120 byusing a magnetic field. In this variation, solid additives 120 include amagnetic metal. The mixture of pellets 100 and solid additives 120enters separator 152 via inlet 154. Magnet 156 generates a magneticfield that pulls solid additives 120 toward the magnet thereby directingthe solid additive into conduit 158. Pellets 100 which are not affectedby the magnetic field are directed toward conduit 160 thereby completingthe separations.

FIGS. 5B and 5C illustrate a method of separation based on densitydifference between pellets 100 and solid additives 120 in a liquid. InFIG. 5B, solid additive 120 floats in liquid 162 and is less dense thanpellets 100. Therefore, pellets 100 and solid additives 120 enteringseparator 164 via conduit 166 are separated by floatation of solidadditives 120 with subsequent exiting via conduit 168. Pellets 100 sinkin liquid 162 and are exited through conduit 170. In FIG. 5C, pellets100 are less dense than solid additives 120 and are removed at the topvia conduit 168 while solid additives 120 sinks and are removed from thebottom via conduit 170.

FIGS. 5D and 5E illustrate a method of separation based on dragdifferences between pellets 100 and solid additives 120. In FIG. 5D,Pellets 100 and solid additives 120 enter separator 180 via conduit 182.A gas (e.g., air) is flowed in from bottom 184 and exits via top 186.Solid additives 120 are push out top 186 while pellets 100 fall towardsbottom 184 and are removed via conduit 188. In FIG. 5E, solid additives120 have a smaller drag coefficient than pellets 100. Therefore, in thisvariation, pellets 100 are driven to top 186 while solid additives 120fall to bottom 184 and are removed via conduit 188.

FIGS. 5F and 5G illustrate a method of separation based on sizedifferences between pellets 100 and solid additives 120. In FIG. 5F,solid additives 120 are larger than pellets 100. Pellets 100 and solidadditive 120 are introduced into separator 190 via conduit 192.Separator 190 includes wall 196 which includes openings 198 that arelarge enough for pellets 100 to pass through but too small for solidadditives 120 to pass through. Pellets 100 fall to the bottom ofseparator 190 and are removed via conduit 200. Solid additives 120 passthrough separator 190 and are removed via conduit 202. In FIG. 5F, solidadditives 120 are smaller than pellets 100. In this variation, solidadditives 120 pass through wall 196 and are removed via conduit 200while pellets 100 pass through to conduit 202.

FIGS. 5H and 5I illustrate another method of separation based on densitydifferences between pellets 100 and solid additives 120. In FIG. 5H,pellets 100 are more dense than solid additives 120. In this variation,pellets 100 and solid additives 120 enter cyclone 210 via conduit 212.Pellets 100 are removed from cyclone 210 via conduit 214 and solidadditives 120 via conduit 216. In FIG. 5I, pellets 100 are less densethan solid additives 120. In this variation, pellets 100 and solidadditives 120 enter cyclone 210 via conduit 212. Solid additives 120 areremoved from cyclone 210 via conduit 214 and pellets 100 via conduit216.

With reference to FIGS. 6A and 6B, schematic illustrations of techniquesthat may be used to convey pellets 100 and solid additives 120 areprovided. As set forth above, a motor may be used to vibrate thecrystallizers of the invention in a manner such that pellets 100 areconveyed from inlet 104 to outlet 106. In FIG. 6A, crystallizer 102includes partitions 220 to 226 that divide the hollow interior ofcrystallizer 102 into sections 230 to 238. Crystallizer 102 is vibratedalong direction d₃, which acts to convey pellets 100 and solid additives120. As pellets 100 fill sections 230 to 238 the vibrations cause someof the pellets near the top to be transported to an adjacent region. InFIG. 6B, a method of conveying the pellets in a spiraling fashion isillustrated. In this technique, conveyor 108 is vibrated along directiond₄ in such a manner to induce a spiraling motion d₅ as pellets 100 andsolid additives 120 are conveyed from inlet 104 to outlet 106. Invariations of these embodiments, the crystallizer may be inclineddownward from inlet 104 to outlet 106 to help pellets 100 and solidadditives 120 to move forward while being vibrated along direction d₄.

In another embodiment of the present invention, a crystallizer forcrystallizing amorphous polymeric pellets is provided. With reference toFIGS. 2, 3, and 4 idealized schematic illustrations of crystallizers ofthis embodiment are provided. Crystallizer 102 includes inlet 104 forreceiving a plurality of polymeric pellets 100. Crystallizer 102 alsoincludes a conveyor 108 for transporting the plurality of pellets 100and solid additives 120 from a first location to a second location. In avariation of this embodiment, conveyor 108 is vibrated by motor 110 insuch a manner that pellets 100 and solid additives 120 move towardspellet outlet 106. Advantageously, conveyor 108 can also agitate pellets100 as they are being conveyed so that sticking or clumping together canbe minimized. Crystallizer 102 also includes solid additive applicator124 for contacting the plurality of polymeric pellets 100 with solidadditives 120. Pellet outlet 106 is used as set forth above for removalof the polymer pellets after crystallization.

With reference to FIG. 7, a schematic side view of a crystallizer thattransports pellets in the manner depicted in FIG. 6A is provided.Crystallizer system 240 includes shaker deck 242 that has top section244 and bottom section 246 which are attached together along joint 250.Together top section 244 and bottom section 246 define crystallizercavity 252. Pellets 100 are introduced into pellet inlet 254 and removedthrough outlet 256 in the manner set forth above. The temperatureadjusting solid additives 120 is introduced via solid additiveapplicator 258. Crystallizer system 240 includes partitions 260 to 270that divide shaker deck 242 into regions 272 to 284. Motor 288 shakesshaker deck 242 along direction d₃, which is substantially along thesame direction as the pellets are conveyed from inlet 254 to outlet 256.In at least the illustrated embodiment, motor 288 is attached to bottomsection 246 of shaker deck 240 via shaft 298 and attachment bracket 300.Crystallizer system 240 includes frame 302, which is attached to bottomsection 246 of shaker deck 242 by springs 304, 306. Springs 304, 306provided the flexibility for vibration of shaker deck 242.

With reference to FIGS. 8A and 8B, schematic illustrations of acrystallizer that transports pellets in a spiraling forward motion asdepicted in FIG. 6B are provided. Crystallizer system 310 includesshaker deck 312, which has top section 314 and bottom section 316 whichare attached together along joint 318. Together top section 314 andbottom section 316 define crystallizer cavity 322. Pellets areintroduced into pellet inlet 324 and removed through outlet 326 in themanner set forth above. The solid additives 120 are introduced via solidadditive applicator 328. Motor 330 shakes shaker deck 312 alongdirection d₄ (FIG. 8B) thereby causing the pellets to move from inlet324 to outlet 326 with a spiraling motion as indicated by d₅. In atleast the illustrated embodiment, motor 330 is attached to the bottomsection 316 of shaker deck 312 via shaft 332. Crystallizer system 310includes frame 336 that is attached to bottom section 316 by springs340, 342. Springs 340, 342 provided the flexibility for vibration ofshaker deck 312. In certain variations, shaker deck 312 is inclineddownward from inlet 324 to outlet 326.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. A method of crystallizing polymeric pellets in a crystallizer havingan inlet, an outlet, and a contact region between the inlet and outlet,the method comprising: a) introducing a plurality of polymeric pelletsinto the crystallizer, the plurality of polymeric pellets comprising acrystallizable polymer having a crystallization temperature and amelting temperature, and the plurality of polymeric pellets having anaverage pellet temperature; b) introducing a plurality of solidadditives into the contact region of the crystallizer, the plurality ofsolid additives having a temperature sufficient to allow at leastpartial crystallization of the plurality of polymeric pellets whilemaintaining the average pellet temperature below the melting temperaturewhile the plurality of pellets is within the crystallizer; and c)removing the plurality of polymer pellets from the outlet of thecrystallizer.
 2. The method of claim 1 wherein the plurality of solidadditives comprise a substantial portion of three-dimensional objectscharacterized by a largest size dimension, with the largest sizedimension being less than 0.25 inches.
 3. The method of claim 2 whereinthe three-dimensional objects have a surface to volume ratio from 20:1to 500:1.
 4. The method claim 1 wherein the solid additives comprise anobject selected from the group consisting of metallic objects, ceramicobjects, glass objects, polymeric objects, and combinations thereof. 5.The method claim 4 wherein the solid additives comprise magneticmetallic objects.
 6. The method claim 4 wherein, after step b), thesolid additives are separated from the plurality of polymeric pellets bydirecting the movement of the solid additive with a magnetic field. 7.The method of claim 1 wherein the solid additives comprise recycledpolymer.
 8. The method of claim 7 wherein the recycled polymer comprisespolyester.
 9. The method of claim 1 wherein the plurality of polymericpellets introduced in step a) comprise regions having a temperaturegreater than or equal to the crystallization temperature of the polymersuch that the plurality of polymeric pellets at least partiallycrystallize while inside the crystallizer.
 10. The method of claim 1wherein the plurality of solid additives has a temperature sufficient toallow at least partial crystallization of the plurality of polymericpellets while maintaining the maximum pellet temperature below themelting temperature while the plurality of pellets is within thecrystallizer.
 11. The method of claim 1 wherein the difference betweenthe average pellet temperature of the pellets in step a) and thecrystallization temperature is less than the temperature rise induced bythe crystallization of the pellets in the absence of cooling during stepb).
 12. The method of claim 1 wherein the plurality of pellets is formedby cutting polymeric strands using a strand cutter.
 13. The method ofclaim 1 wherein the plurality of pellets are formed by cutting polymericstrands using a die face pellet cutter.
 14. The method of claim 1wherein the polymeric pellets, after step c), have a degree ofcrystallinity equal to or greater than 30%.
 15. The method of claim 1wherein the polymeric pellets, after step c), have a degree ofcrystallinity equal to or less than 70%.
 16. The method of claim 1wherein the polymeric pellets, after step c), have a degree ofcrystallinity equal to or greater than 40%.
 17. The method of claim 1wherein a portion of the plurality of polymeric pellets arethree-dimensional objects characterized by a largest size dimension,with the largest size dimension being less than 0.25 inch.
 18. Themethod of claim 1 wherein a portion of the plurality of polymericpellets have a spherical shape.
 19. The method of claim 1 wherein aportion of plurality of pellets have a rectangular cross section. 20.The method of claim 1 wherein a portion of the plurality of polymericpellets have a cylindrical shape.
 21. The method of claim 1 wherein thepolymeric pellets comprise a component selected from the groupconsisting of polyester, polyolefins, polystyrenes, nylons, andpolyketones.
 22. The method of claim 1 wherein the plurality ofpolymeric pellets comprise polyethylene terephthalate.
 23. The method ofclaim 22 wherein the temperature range is from about 135° C. to about200° C.
 24. The method of claim 1 further comprising the step of: d)separating the plurality of polymer pellets from the plurality of solidadditives.
 25. The method of claim 24 wherein step d) is performed priorto step c), after step c), or during step c).
 26. The method of claim 24wherein, after separation of the solid additives from the plurality ofpolymeric pellets, the solid additives are reused by the methodcomprising: e) adjusting the temperature of the solid additives; and f)re-introducing the solid additives into the contact region of thecrystallizer, the solid additives having a temperature sufficient toallow at least partial crystallization of the plurality of polymericpellets while maintaining the average pellet temperature below themelting temperature while the plurality of pellets is within thecrystallizer.
 27. The method of claim 24 wherein the plurality ofpolymeric pellets are separated from the solid additives by centripetalforce.
 28. The method of claim 24 wherein the plurality of polymericpellets are separated from the solid additives by drag differencebetween the solid additives and the plurality of polymeric pellets. 29.The method of claim 24 wherein the plurality of polymeric pellets areseparated from the solid additives by size difference between the solidadditives and the plurality of polymeric pellets.
 30. The method ofclaim 24 wherein the plurality of polymeric pellets are separated fromthe solid additives by density difference between the solid additivesand the plurality of polymeric pellets.
 31. The method of claim 1wherein the plurality of polymeric pellets are agitated while beingconveyed.
 32. The method of claim 31 wherein the plurality of polymericpellets are agitated by directing the plurality of pellets to spiral.33. The method of claim 1 wherein the crystallizer includes one or moresides that are partially or completely insulated.
 34. The method ofclaim 1 wherein the crystallizer includes one or more sides that arecompletely uninsulated.
 35. The method of claim 1 wherein the pluralityof pellets are introduced into the crystallizer in an amount of 5,000lb/hr to 200,000 lb/hr.
 36. The method of claim 1 wherein the pluralityof pellets have an average residence time from 1 second to 1 hour. 37.The method of claim 1 wherein the plurality of pellets have an averageresidence time from 1 minute to 10 minutes.
 38. The method of claim 1wherein the solid additives comprise a sublimeable solid.
 39. The methodof claim 1, further comprising, in a continuous process, melt phasepolymerizing virgin polyester molten polymer, solidifying the moltenpolymer to form pellets in contact with water, separating at least aportion of the water from the pellets, and introducing said pellets intosaid convey system.
 40. The method of claim 39 wherein said polyesterpolymer pellets formed from the solidification process have an It.V. ofat least 0.70 dL/g.
 41. The method of claim 40 wherein the It.V. is atleast 0.72 dL/g.
 42. The method of claim 41 wherein the It.V. is atleast 0.76 dL/g.
 43. A crystallizer for crystallizing polymeric pellets,the crystallizer comprising: an inlet for receiving a plurality ofpolymeric pellets; a conveyor for transporting the plurality of pelletsfrom a first location to a second location, the conveyor agitating thepellets as the pellets are being conveyed; a solid additive applicatorfor contacting the plurality of polymeric pellets with a plurality ofsolid additives; and an outlet for removing the plurality of polymerpellets.
 44. The crystallizer of claim 43 having an open top section.45. The crystallizer of claim 43 having a closed top section.
 46. Thecrystallizer of claim 43 wherein the conveyor transports the pellets byagitation.
 47. A method of crystallizing PET pellets in a crystallizerhaving an inlet, an outlet, and a contact region between the inlet andoutlet, the method comprising: a) introducing a plurality of PET pelletsinto the crystallizer, the plurality of PET pellets being crystallizableand having a crystallization temperature and a melting temperature, andthe plurality of polymeric pellets having an average pellet temperature;b) introducing a plurality of solid additives into the contact region ofthe crystallizer, the plurality of solid additives having a temperaturesufficient to allow at least partial crystallization of the plurality ofPET pellets while maintaining the average pellet temperature below themelting temperature while the plurality of pellets is within thecrystallizer; and c) removing the plurality of PET pellets from theoutlet of the crystallizer.